The invention relates generally to wastewater treatment for the purification of municipal sewage and more particularly to the treatment and further purification of the outflow of a municipal sewage treatment plant which performs primary and secondary treatment.
Treatment of municipal sewage is often described in a hierarchy of treatment levels. Primary treatment generally involves settling out suspended solids. Secondary treatment employs microbes which digest many of the pollutants. Tertiary treatment most generally involves a final filtration step as well as further treatment to render the effluent virtually free of pollutants. Many municipalities have wastewater treatment plants which provide primary and secondary treatment. Because of high cost, tertiary treatment is uncommon.
Heretofore, secondary treatment has been sufficient to meet state and federal pollution control regulations. Under new stricter standards being proposed and implemented, secondary treatment will not always meet the pollution control requirements for discharge into rivers and streams. Municipalities are being forced to seek methods and techniques for further purifying secondary treated wastewater. Tertiary treatment meets and exceeds pollution control requirements, but is prohibitively expensive when state-of-the-art mechanical filtration systems are used. Thus, municipalities continue to seek improved, more economical methods for treating secondary treatment outflow
One known technique for purifying wastewater employs artificial wetlands containing hydrophytes such as reeds and other marsh plants. Wastewater which is introduced at or near the surface level of the ground, undergoes purification through gravity filtration and then the purified wastewater is collected substantially below the ground surface. Such methods for treating wastewater are shown in U.S. Pat. No. 4,839,051 to Higa, U.S. Pat. No. 4,824,572 to Scott and U.S. Pat. No. 4,678,582 to Lavigne.
Wetland environments cleanse polluted water, but have several disadvantages when used to treat municipal wastewater A medium-to-large city might require tens or hundreds of acres of artificial wetlands to adequately treat its secondary treated wastewater. Acreage covered with artificial wetlands is neither usable for human activities, nor easily accessible except on pathways constructed through the wetlands Treatment ponds in the form of wetlands are also often aesthetically problematic, due to unpleasant odors, and attraction of insects and the like.
There is and will continue to be an increasing need for effective, economically practical systems for purifying secondary treatment outflow from wastewater treatment plants. There is also a need for providing such treatment in or near cities in a manner which is compatible with nearby human occupation and activities. There is a further need for providing such wastewater treatment in a way which reduces both the quantity of wastewater and the pollutants it carries.
It is an object of the present invention to provide a system for treating wastewater which has received primary and secondary treatment, such wastewater being referred to as secondary treated wastewater, by reducing its volume or quantity and by cleansing the wastewater.
It is a further object of the invention to provide a system for treating secondary treated wastewater by means of relatively large biological treatment beds, extending over one or more acres of ground, which are substantially dry, hard surfaces capable of supporting wheeled vehicular traffic and which are implanted with turf grass capable of biologically cleansing the wastewater.
It is another object of the invention to provide a system for treating secondary treated wastewater using an in-ground treatment bed by extracting the cleansed wastewater from below the surface of the ground after anti-gravity flow, leaving the surface of the treatment bed substantially dry and hard.
It is yet a further object of the invention to provide a method of treating secondary treated wastewater which employs grass-covered beds of rock media which are sufficiently stable to support wheeled vehicular traffic, the beds being filled with rock pieces having a size and shape which produces voids therebetween through which liquids, including the wastewater being treated, can pass.
Accordingly, the present invention provides a near ground surface water treatment system for treating wastewater comprising a water-impervious basin which has a base, sides and an open top. The system also includes a treatment water injection system adjacent the base of the water-impervious basin. Means are provided for withdrawing treated water from the basin at a point above the water injection system. A water table is defined by the treated water withdrawal means, the water table being below the top surface of the basin. And treatment elements, preferably in the form of turf grass roots extending into the basin, are operatively interposed between the injection and withdrawal means.
In a preferred embodiment of the invention, a waste-water inlet channel is provided near the bottom of the bed and includes a pattern of perforated tubing extending through the media. An outlet channel is provided near the top of the bed. A large proportion of the outflow withdrawn from the bed near the top is recirculated back through the inflow inlet channel, where it again passes through the voids in the bed. About five-to-ten percent of the inflow into the bed is new, secondary treated wastewater from the treatment plant. In order to sustain and encourage root growth in the voids of the media, a system for aerating the wastewater in the bed is provided. The aeration system is designed to maintain a dissolved oxygen content of between about one and about seven parts-per-million in the wastewater in the bed.
Another form of the invention is a method of treating secondary treated wastewater, namely, wastewater which has received primary and secondary treatment in a wastewater treatment plant. A first step in the method is to inject waste-water at a predetermined subsurface level into a bed of rock media enclosed at the sides and bottom by a water-impervious barrier. The bed in which the wastewater is injected is formed of rock pieces with voids therebetween through which liquids are able to flow. The bed includes turf grass growing on top, with roots which extend down into the bed in the voids of the media. The next step in the method is to saturate the be with wastewater and then to withdraw wastewater from the bed at a higher level than the level at which the wastewater is injected. The preferred method also includes the step of aerating the wastewater in the bed as it is circulated through the bed. Aeration encourages and sustains root growth in the voids of the media. The wastewater circulating through the bed is treated and partially evaporated while providing water and nutrients to the roots of the turf grass. Consequently, the wastewater withdrawn from the bed at the higher level is of a greater purity and has a smaller volume than the wastewater initially injected into the bed.